1. Field of the Invention
The present invention relates to a power supply cable preferable for use in a vehicle to supply electric power from the battery of the vehicle to each section thereof. More specifically, the present invention relates to a power supply cable having "a semiconductor active fuse" which can detect an abnormal current and/or short-circuit failure and if necessary, can cut off the conduction of the current. The present invention also relates to a power supply system having a plurality of the power supply cables.
2. Description of the Related Art
FIG. 1 is a diagram showing a conventional power supply system provided with an overcurrent controller installed in a vehicle, where electric power is selectively supplied from the battery to each load of the vehicle, and a transistor QF, having a temperature sensor, controls the supply of the electric power. In the conventional overcurrent controller shown in FIG. 1, the power supply cable extending from a power source 101 for supplying an output voltage VB is connected to an end of a shunt resistor RS and a drain terminal D of the transistor QF, having the temperature sensor, is connected to the second end of the shunt resistor RS. To a source terminal S of the transistor QF, a load 102 is connected. The load 102 is, for example, a headlight or a driving motor for power windows of the vehicle. The overcurrent controller used for the power supply cable shown in FIG. 1 further includes a driver 701 for detecting a current which flows through the shunt resistor RS and controlling the operation of the transistor QF, and A/D converter 702 for converting the analogue value of the current monitored by the driver 701 into the digital values, and a microcomputer (CPU) 703. The transistor QF has a thermal protection function such that it can turn off when its junction temperature increases to a predetermined temperature or higher. In this case, a temperature sensor incorporated therein detects the increase of the temperature of the semiconductor chip, and then a flip-flop circuit (or a latch circuit) is triggered by the temperature sensor to turn on the thermal cutoff transistor which cause the transistor QF compulsorily to be turned off.
In FIG. 1, a Zener diode ZD1 keeps the voltage between the gate terminal G and the source terminal S of the transistor QF at 12 V. When an overvoltage is applied to a true gate TG of the transistor QF, the Zener diode ZD1 bypasses the overvoltage. The driver 701 includes differential amplifiers 711, 713 as current monitors, a differential amplifier 712 as a current limiter, a charge pump 715, and a driver 714. The driver 714 drives the true gate TG of the transistor QF through an internal resistor RG based on an ON/OFF control signal from the microcomputer 703 and the result of the judgment from the differential amplifier 712 as the current limiter whether or not an overcurrent is generated. When the differential amplifier 712 detects the over current more than the predetermined, by means of the voltage drop across the shunt resistor RF, the driver 714 turns off the transistor QF. After that, when the current value decreases less than a lower criterion, the driver 714 turns on the transistor QF, again. The microcomputer 703 always monitors the current via the current monitors (i.e. the differential amplifiers 711, 713). When the flow of an abnormal current exceeding the normal value is monitored, the microcomputer 703 outputs a signal for turning off the transistor QF. Upon receiving the signal, the transistor QF is turned off. However, if the temperature sensor installed in the transistor QF detects that the temperature exceeds the predetermined value before the microcomputer 703 outputs the signal for turning off the transistor QF, the temperature sensor outputs a signal for making the thermal protection function of the transistor QF to work. Upon receiving the signal, the transistor QF is turned off.
As described above, the overcurrent controller employed for the conventional power supply cable requires the shunt resistor RS connected to a power supply path in series in order to detect the current. Thus structure, however, has a problem as follows. Since larger and larger current flows in the load in recent years, it becomes impossible to ignore the heat dissipation of the shunt resistor, and the conduction loss of the power supply cable becomes large. This problem is especially serious when larger current is passed through the power supply cable, and in such a case, it is required to install a cooler to the overcurrent controller.
In addition, there is another problem. That is, in the case of using the conventional power supply cable having the thermal protection function, the over current control circuit successfully works when the load 12 or the power supply cable would be short-circuited completely so that a large current flows therethrough. However, such functions do not work when a layer short-circuit, or an imperfect short-circuit having some degree of short-circuit resistance occurs so that a low short-circuit current flows through the power supply cable. As the only method to cover this drawback, the monitor must diagnose the current flowing through the power supply cable so that the microcomputer 703 recognizes an abnormal current. When the microcomputer 703 detects an abnormal current, it turns off the transistor QF. However, this method has a problem that, the current detection of the microcomputer 703 is not so fast that shutting down of the transistor QF may be delayed.
There is also a problem that, in accordance with the necessity to mount the shunt resistor RS, the A/D converter 702, and the microcomputer 703 or the like, a large space is required for the power supply device. In addition, since these elements are relatively expensive, the cost of the apparatus provided with them becomes high.